Searching for large-scale structures around high-redshift radio galaxies with Herschel

Abstract

This paper presents the first results of a far-infrared search for protocluster-associated galaxy overdensities using the SPIRE instrument on-board the {\it Herschel} Space Observatory. Large ($\sim$400 arcmin${}^2$) fields surrounding 26 powerful high-redshift radio galaxies ($2.0<z<4.1$; $L_{500MHz}>10^{28.5}$ WHz${}^{-1}$) are mapped at 250, 350 and 500\mic to give a unique wide-field sample. On average the fields have a higher than expected, compared to blank fields, surface density of 500\mic sources within 6 comoving Mpc of the radio galaxy. The analysis is then restricted to potential protocluster members only, which are identified using a far-infrared colour selection; this reveals significant overdensities of galaxies in 2 fields, neither of which are previously known protoclusters. The probability of finding 2 overdensities of this size by chance, given the number of fields observed is $5\times 10^{-4}$. Overdensities here exist around radio galaxies with $L_{500MHz}\gtrsim 10^{29}$ WHz${}^{-1}$ and $z<3$. The radial extent of the average far-infrared overdensity is found to be $\sim$6 comoving Mpc. Comparison with predictions from numerical simulations shows that the overdensities are consistent with having masses $>10^{14}$Msolar. However, the large uncertainty in the redshift estimation means that it is possible that these far-infrared overdensities consist of several structures across the redshift range searched.

13 pages, 9 figures; Accepted for publication in MNRAS